This invention relates to measuring the concentration of nitrogen oxide compounds (NO.sub.x) in a gaseous mixture using a solid state sensing element comprising a semiconductive zinc oxide thin film.
The emission of gaseous nitrogen oxide compounds, particularly from automotive internal combustion engines, is a major environmental concern. Several nitrogen oxide compounds have been identified and are generally referred to as a group by the symbol "NO.sub.x ". The most significant are nitric oxide (NO) and nitrogen dioxide (NO.sub.2).
To better study and monitor NO.sub.x emissions, an instrument is desired that provides accurate, continuous readings directly from a sample gas without pretreatment or reagents and without interference from other sample constituents. It is known that the electrical resistance of a thin film of a semiconductive material exposed to a gaseous mixture may be affected by the presence of certain species, depending upon the material, the species and the composition of the mixture. U.S. Pat. No. 4,169,369, issued to the present inventor and coassigned, describes a NO.sub.x sensor having a semiconductive tin oxide thin film. Zinc oxide, iron oxide, lead oxide and cadmium oxide also form N-type thin film semiconductors that are sensitive to common reducing and oxidizing gaseous species in an inert gas. However, it is not possible to predict the effect of a particular species upon the thin film in the presence of a relatively high concentration of another species to which the film is sensitive. In contrast to tin oxide, it has been found that iron oxide, lead oxide and cadmium oxide are essentially insensitive to NO.sub.x in air. Apparently, the relatively high oxygen concentration saturates the thin film surface so that any interaction with NO.sub.x species is too small for detection. In view of this, it is totally unexpected that a zinc oxide sensor could be adapted to measure NO.sub.x concentration in the presence of a relatively high oxygen concentration, such as in air, or other oxidizing or reducing species.
It is an object of this invention to provide a method for detecting NO.sub.x in a gaseous mixture utilizing a semiconductive zinc oxide thin film to make relatively rapid, direct readings without sample pretreatment or reagent additions, which readings produce an accurate measurement of the NO.sub.x concentration despite a relatively large concentration of oxygen in the mixture and further despite the presence of other oxidizing and reducing species, including hydrogen and propylene, in the mixture.